Evolutionarily Stable Strategies in Guam
The brown tree snake is an invasive species that has devastated bird species in Guam and has destroyed the whole island’s ecosystem. The snake was likely introduced into Guam after being introduced by US military equipment transportation and has caused the endangerment of Guam’s bird and rodent species. The snakes were first introduced to the island around Guam’s liberation in 1944, and “by the mid-1980s, 10 of the 12 bird species native to Guam had vanished, including a kingfisher that can’t be found anywhere else on Earth”. The reason that the snake has been so devastating to the wildlife on Guam is that most of the wildlife has evolved without predators like the brown tree snake that can eat their eggs and young. This situation can be modeled as two tables, one of the wildlife of Guam before the introduction of snakes and after the introduction.
| Bird Species 2 | |||
| Protect Eggs | Don’t Protect Eggs | ||
| Bird Species 1 | Protect Eggs | 0, 0 | 0, 1 |
| Don’t Protect Eggs | 1, 0 | 1, 1 | |
The first table represents the bird species on Guam before the introduction of snakes. As there is no predator on the island, the bird species that protect their eggs have a payoff of 0 as they are spending energy on protecting their eggs for no reason. If the bird instead uses that energy on other activities such as finding more food, then it has a payoff as 1. The expected payoff of a bird protecting their eggs is (1-x)0+x0 = 0-0, which is around 0. The expected payoff for a bird not protecting their egg is (1-x)1 + x1 = 1-x+x = 1. This means that A is an ESS in this situation.
| Bird Species 2 | |||
| Protect Eggs | Don’t Protect Eggs | ||
| Bird Species 1 | Protect Eggs | 1, 1 | 1, 0 |
| Don’t Protect Eggs | 0, 1 | 0, 0 | |
The second table represents the bird species on Guam after the introduction of snakes. As there is now a predator that hunts their birds and eggs, the bird species that protect their eggs have a payoff of 1 as they are able to protect their eggs from the snakes and increase their fitness, and birds that do not protect their eggs have their eggs eaten by the snakes and get a payoff of 0. This means that after the introduction of the brown tree snake, the ESS for a bird is to protect their eggs.
Even though it is an ESS for a bird to protect their eggs after the introduction of the brown tree snake, this introduction has caused the disappearance of 10 out of 12 native bird species because the birds had all evolved according to the first table. Without the time needed for evolution, the birds are all unable to protect their eggs and disappear from the island due to the new invader.
https://www.sciencealert.com/guam-s-plague-of-snakes-is-having-a-devastating-impact-on-the-trees